Why Am I Always Tired? – Mental Fatigue vs Physical Fatigue

We’ve all had those days where, despite getting a full night’s sleep, we still wake up groggy, sluggish, and drained. The yawn becomes a loyal companion, motivation is nowhere to be found, and even the simplest tasks feel like climbing mountains. If you’ve caught yourself asking, “Why am I always tired?” — you’re not alone.

The answer, more often than not, lies in the distinction between mental fatigue and physical fatigue. And while they can go hand in hand, understanding the difference is key to finding the right remedy. It is usually localised and measurable.

What is Physical Fatigue?

Physical fatigue is the kind of tiredness we usually associate with physical exertion. It’s what your body feels after a long hike, an intense workout, or even a full day of running errands.

Muscle fatigue is a complex, multifaceted phenomenon and is most commonly defined as the inability to maintain a required force level after prolonged use of the muscle. (Parasuraman, 2014) 

Symptoms of Physical Fatigue

• Aching muscles

• Slowed reflexes

• Low stamina

• A strong desire to sleep

  
  

According to Ranjana K. Mehta and Raja Parasuraman, overt symptoms of physical fatigue include:

• Inability to maintain required force levels – muscles can no longer sustain the needed strength during tasks.

• Decrease in maximal strength – evident from lower post-exercise handgrip strength compared to baseline.

• Increased joint fluctuations – reduced joint steadiness and control (more tremors or shaking).

• Increased muscle activity (EMG RMS) – muscles need to work harder to perform the same task.

• Decreased muscle frequency output (EMG MdPF) – a physiological marker indicating muscle fatigue progression.

• Subjective discomfort – higher discomfort and perceived exertion as the task progresses.

• Slower performance or inability to complete physical tasks – reaching exhaustion and dropping below target effort levels.

  
  

  Physical fatigue often improves with proper rest, hydration, and nutrition. It’s your body’s way of saying, “I need a break.” However, if it persists despite adequate recovery, it might be a sign of something deeper — a vitamin deficiency, hormonal imbalance, or chronic illness.

What is Mental Fatigue?

Now, here’s where things get a bit trickier. Mental fatigue is more invisible, more insidious. It sneaks up after long hours of concentration, emotional strain, decision-making, or just constantly being “on.” According to Mehta and Parasuraman, Prolonged periods of cognitive processing lead to manifestation of mental fatigue, which is commonly determined by subjective fatigue assessments or decrements in performance.

Symptoms of mental fatigue

• Feeling emotionally drained

• Trouble concentrating or making decisions

• Irritability or low mood

• A sense of detachment or disinterest

• Overwhelm, even in minor situations

This form of exhaustion often isn’t fixed by sleep alone — because it’s not just your body that’s tired, it’s your mind. Mental fatigue can stem from ongoing stress, academic or work pressure, personal responsibilities, or even emotional burnout.

How are mental and physical Fatigue interrelated?

• Mental fatigue impairs motor performance: Even without direct physical exertion, prolonged cognitive effort can lead to reduced physical function.
  

• Neurological interference: When performing both cognitive and physical tasks, the brain struggles to allocate resources, resulting in faster exhaustion.

• When mental and physical fatigue occur together, even though physical performance may look the same on the outside, the brain shows clear signs of overload.
  

• Physical fatigue can lead to mental exhaustion: When your body’s worn out (from lack of sleep, illness, overexertion), your brain has to work harder to function normally. You might feel foggy, irritable, or less motivated—classic signs of mental fatigue.

  
  

• Stress & Overthinking = Tired Body: Stress hormones like cortisol keep your body tense, heart rate up, and sleep quality down. If you’re constantly thinking or worrying, your nervous system never really "shuts off," and that drains physical energy—even if you’ve been sitting all day.

• Poor Sleep = Sluggish Brain: If your body doesn’t get proper recovery (especially deep sleep), your brain struggles with focus, memory and emotional regulation. This is why sleep-deprived people often feel both physically and mentally wiped.

• Emotional Fatigue = Whole-Body Burnout: Carrying emotional weight (grief, depression, anxiety) can feel like walking with bricks in your pockets. Your body literally slows down in response to emotional stress. You might feel muscle aches, tension, or just “heavy,” even without exerting yourself.

•  Lack of Movement = More Fatigue: Oddly enough, not moving much can cause both types of fatigue, too.

  • Your circulation slows

  • Endorphins drop

  • Muscles get tight

  • Your mind gets foggy